The present invention relates to a method and arrangement for making a web adhere to a drying wire in dryer groups provided with single-wire draw in a multi-cylinder dryer of a dryer section of the paper machine so as to prevent or at least substantially curtail the effects of centrifugal forces and other air-flow phenomena which attempt to separate the web from the drying wire in the dryer groups. The dryer groups comprise drying cylinders which are heated by means of steam or equivalent, and the web is pressed therein against outer, heated cylinder faces of the drying cylinders by means of the drying wire. The dryer groups further comprise guide cylinders or rolls on which the web runs on the outside face of the drying wire. The guide cylinders or rolls are provided with perforations which pass through the cylinder mantle and open into the inside space in the interior of the guide cylinder or roll, the inside space being subjected to a vacuum. The vacuum in the guide cylinders or rolls is transferred to the outside face of the drying wire through the perforations, and the guide cylinders or rolls comprise shafts on which the guide cylinders or rolls are revolvingly mounted.
In the prior art, multi-cylinder dryers of paper machines usually consist of two horizontal rows of drying cylinders placed one row above the other whereby a paper web runs a meandering course between the rows. At present, in connection with the drying cylinders, drying wires penetrable by air are used, i.e. permeable drying wires.
In the dryer section of a paper machine, single-wire draw and/or twin-wire draw is/are employed. Single-wire draw refers to a draw in which the web runs from one drying cylinder onto another cylinder, which are typically in two different rows, on support of one and the same drying wire as well as on support thereof across the gaps between the rows of cylinders. In twin-wire draw, an upper wire and a lower wire are employed to press the web against heated cylinder faces of drying cylinders in an upper row and a lower row, respectively, and the web has free, unsupported draws when it runs from one row of drying cylinders to the other.
The present invention is primarily related to a single-wire draw dryer group in which the web runs from one row of cylinders to the other on support of the same drying wire so that, on one row of cylinders, the web is situated between the drying wire and the cylinder face and, on the other row of cylinders, the web is situated farthest from the cylinder, the drying wire being situated between the cylinder face and the web, and the web runs over the draws between the rows of cylinders on support of the drying wire. It is an advantage of this single-wire draw that the web is constantly supported by the drying wire, and has no free draws, whereby the risk of wrinkles in and breaks of the web are reduced.
In prior art multi-cylinder dryers that apply single-wire draw, on the cylinders on which the web to be dried is placed outside or farthest from the cylinder, which are conventionally leading rolls or cylinders, a drawback is a tendency of separation of the web from the face of the drying wire by the effect of centrifugal force, which phenomenon is intensified further by the pressures induced in the inlet nip between the drying wire and the cylinder faces. This separation already produces breaks or at least bag formation and wrinkles in the web.
At the reversing rolls in these groups with single-wire draw in the dryer section of a paper machine, a problem is the adhering of the paper web to the wire on the lower face of the cylinder. 0n the lower cylinders, i.e., the reversing rolls, the web runs topmost, i.e., the farthest from the cylinder faces, while the wire remains between the web and the outer face of the cylinder. In the case of the other rolls in a group with single-wire draw, this problem does not occur since the paper web runs between the wire and the mantle face of the cylinder.
Attempts have been made to solve this problem by using suction rolls in which there are separate suction sectors in the interior. A suction-roll solution is, however, expensive and requires servicing. A more favorable solution is described in the assignee's Finnish Patent No. 83,680 (corresponding to U.S. Pat. No. 5,022,163), in which a cylinder for a paper machine is described which is particularly suitable for use for supporting the web in connection with the lower rolls in a single-wire group. This cylinder is mainly characterized in that the grooved cylinder includes numerous holes which pass through the mantle and one of whose ends open in the grooves and the other end into the interior space in the cylinder. This grooved roll is provided with drilled suction holes passing through the roll mantle to the bottom of the grooves in the roll, and one end or both ends of the roll is/are provided with a shaft which comprises a suction duct, which communicates further with a source of suction. When the holes are dimensioned suitably and when the roll mantle is provided with a certain limited number of bores that transfer the vacuum (negative pressure or suction force), a permanent vacuum is provided in the interior of the roll. This vacuum can be maintained in spite of the fact that some of the holes in the roll are opened into the open air in the top portion of the roll. The effect of the vacuum spreads in the groove. In this manner, a simple roll solution has been provided in which, by means of the vacuum, the web is pulled toward the roll and the suction is applied to the web through the wire.
With respect to other prior art related to the present invention, reference is made to European Patent Application No. 94 200 656 (EP 0 617 162), which claims the priority of Finnish Patent Application No. 881106 which in turn corresponds to the above-mentioned Finnish Patent, and was separated from the European patent application corresponding to above-mentioned Finnish Patent by splitting. In the separated EP application, a method is suggested for making the web adhere to the drying wire so as to prevent the effects of centrifugal forces and other air-flow phenomena, which effects attempt to separate the web from the wire in dryer groups that are provided with single-wire draw in a multi-cylinder dryer of a paper machine. The dryer groups comprise drying cylinders as the upper cylinders, which drying cylinders are heated with steam or equivalent, and the web is pressed against the outer cylinder face of the drying cylinders by means of a drying wire. Further, the dryer groups comprise guide cylinders or rolls as the lower cylinders, on which cylinders the web runs outside the drying wire. The lower cylinders are provided with holes which pass through the cylinder mantle and open into the space in the interior of the cylinder in which a vacuum is present. The vacuum or suction force is transferred to the outside face of the lower cylinders through the holes. The lower cylinders comprise shafts on which the cylinders are revolvingly mounted and at least one of the shafts is provided with a duct through which the vacuum is produced in the interior space in the cylinder. The shafts are connected to a suction duct which communicates with a source of vacuum. In that invention, one of the novel aspects is that a major part of the holes in the cylinder are open toward the open air (atmosphere) in the top portion of the cylinder so that the air flow is directed toward the interior of the cylinders. Another aspect is that the vacuum inside the cylinder is in the range of about 1000 Pa to about 3000 Pa and is maintained substantially constant.
It has been a problem in the prior art suction rolls that, in spite of the large amount of air, the vacuum in the rolls has not always been adequate to hold the web during tail threading because of the open perforated face. On the other hand, a web of full width does not necessarily require an equally high vacuum and amount of air as the narrow leader strip does because, when a paper web of full width is transferred, the suction roll operates with a smaller amount of air. Thus, it is an important object of the present invention to suggest a solution to this problem, i.e., to provide a variable arrangement which adjusts the amount of suction depending on the requirements of the situation, e.g., threading stage or the running stage. As a solution for this problem, suction rolls have been suggested in which a separate tail threading zone is employed inside the roll in order to apply an adequate suction effect to the leader strip. In this connection, with respect to the prior art, reference should be made, e.g., to U.S. Pat. Nos. 5,031,338, 4,980,979, 4,905,379, 5,152,078 and 5,015,336. Problems of these rolls have involved their complex construction and high cost as well as abundant requirement of servicing, for example, in order to ensure the operation of the valve inside the roll.
By means of the vacuum employed in the reversing cylinders, it is also possible to affect the shrinkage of the web in the dryer section. In this respect, reference is made to the U.S. Pat. No. 5,241,760, in which it is stated that, owing to the longitudinal shrinkage of the paper web, the web is tensioned in the dryer section and thereby transverse shrinkage of the web is already inhibited on the grooved rolls. Regarding the prior art related to this, reference is also made to the paper Pulkowski et al, "Operating results with the bel-champ single-tier dryer", TAPPI Proceedings, 1990 Engineering Conference, pages 393-398. In this paper, it is stated that, based on test results, it has been found that forced drying provides the advantages of increased modulus of elasticity and strength properties and reduced stretch.
In relation to the effects of shrinkage of the web during drying, it can also be stated that the assignee has carried out tests with production machines, which tests have indicated that a fully single-wire draw of the Sym-Run.TM. type lowers the transverse shrinkage to about one half (e.g., from about 3% to about 1.5%). In the test, when the suction was eliminated from all the suction rolls, the transverse shrinkage was changed from about 1.5% to about 1.8%, i.e., under these circumstances, it can be stated that the effect of the single-wire draw on the change was in this case about 75% to about 80%, and the effect of the suction rolls about 20% to about 25%.
Adherence of the web to the wire on the outer faces of the reversing cylinders or equivalent is also affected by the running speed employed in the dryer section of the paper machine and, of course, by the nature of the paper web that is being run. In such a situation, a constant vacuum in the reversing rolls has not always been suitable in view of holding the web and in view of the shrinkage of the web during drying. For example, it has been noticed that an excessive inhibition of transverse shrinkage of the web has a detrimental effect on the transverse stretch potential of the paper. In some cases, this has even resulted in tearing of the web in the final portion of the dryer section.